This invention relates generally to improvements in vessels equipped with independent pressurized tanks resting against contiguous coverings or shells and, in particular, to those vessels used in the transport of low temperature liquid gases, such as natural gas.
The use of natural gas, greatly in demand today as a source of energy, is limited by the problems associated with its transfer from sources located in the Middle East to the consumer countries.
Until the present time the means used to transport natural gas are based on vessels equipped either with large holds that enclose membranous containers resting along the bottom and the walls of said holds, or, with large, independent tanks resting against contiguous coverings which, in turn, transmit the loads to the hull of the ship.
The use of membranous containers requires a highly specialized work crew and special materials and technology for their manufacture, and monopolizes dockyard facilities for long periods of time.
In comparison, the independent load tanks do not necessitate the use of highly specialized crews or materials for their manufacture and also involves shorter periods of work time. Although the materials required are not so specialized as those required by the membranous containers, they are of considerable weight due to the thickness of the plates necessary for their construction. The thick plates are a necessary factor so as to ensure the stability of the tank wall. In an independent tank resting against contiguous coverings the stability of the wall may be endangered by forces of compression that give rise to pressures greater than or equal to the maximum levels.
Compression pressures in the tanks in question are due to forces of inertia when the vessel is seabound, to the weight of the tank and its load, and to deformations in the vessel hull due to waves.
Deformations in the hull of the ship may require a tank wall to be up to 30% thicker than the width already required by the two aforementioned causes.
It is impossible to reduce the influence of gravity and almost impossible to reduce the forces of inertia. However, it is possible to reduce the pressures on the tank wall due to deformations of the hull, and consequently the width of said wall and the weight of the tanks.
At the present time one method to reduce these pressures is known. It was developed by the applicants and/or assignees of the present invention and consists of controlling and maintaining deformations to the hull within predetermined limits so that they do not rise to high levels.